More than 100,000 Syrians are trapped on the Turkish border in the Aleppo province, as fighters from the Islamic State of Iraq and the Levant (ISIL) group advance in rebel-held territory.

Doctors Without Borders (MSF) evacuated most of its staff and patients from the al-Salamah hospital - the organisation's largest facility in Syria - near the town Azaz on Friday as ISIL neared.

"We are terribly concerned about the fate of our hospital and our patients, and about the estimated 100,000 people trapped between the Turkish border and active frontlines," Pablo Marco, MSF operations manager for the Middle East, said in a statement .

"For some months, the frontline has been around seven kilometers away from the hospital. Now it is only three kilometers from al-Salamah town. There is nowhere for people to flee to as the fighting gets closer."

ISIL has managed to cut off the road connecting the rebel-held towns of Azaz and Marea.

Stefanie Dekker, reporting from the Turkish border town of Kilis, said ISIL has clashed with Free Syrian Army fighters and other opposition groups.

"There are plumes of smoke occurring randomly," she said, describing the scene from across the border and noting the war planes flying above.

"These areas are held by the opposition; they have a lot of civilians, a lot of internally displaced people who have had to move throughout this war from place to place," Dekker said. "It is an active frontline all across Syria."

Journalist Maamoun Khateeb told AFP 15,000 people were now besieged in Marea. "This is a disaster," he said.

Turkey has closed its border to all but seriously injured Syrians.

Marea and Azaz both fell to opposition forces in 2012 and have been vital stops along a rebel supply route from Turkey.

ISIL has tried to advance on both towns for months. In a statement on Friday, the group said it launched a "surprise attack" and seized a series of villages near Azaz.

Also on Friday, government bombardment on rebel-controlled areas of Aleppo province left at least 15 people dead, rescue workers told AFP.

At least two people were killed in barrel bomb attacks on an opposition-controlled eastern district of Aleppo city, the civil defence - known as the White Helmets - said.

Air strikes also killed nine people in the town of Hreitan and four in Kfar Hamra.

Since fighting intensified there in 2012, Aleppo province has been transformed into a patchwork of territories held by the government, rebels, Kurds and other fighters.

Gerry Simpson, a researcher at Human Rights Watch, said Turkey should open the border and allow safe passage to those fleeing the ISIL offensive.

"The fact Turkey is generously hosting more than 2.5 million Syrians does not give it a right to shut its border to other endangered Syrians," he wrote in a HRW statement on Friday.

The UK-based Syrian Observatory for Human Rights estimates that more than 270,000 people have been killed throughout the five-year war.

More than 4.8 million Syrians have become refugees in neighbouring countries - Lebanon, Jordan, Turkey and Iraq - as well as Europe, according to the UNHCR. At least 7.6 million Syrians are internally displaced within the country's borders.